Hi guys I'm running a pa 61 with a rear muffler using a kaz minato carbon fibre clank tank ,the problem I'm having is it starts off with a nice two four break but Slowly richens up though the pattern by the time I get to the overheads it's a four stroke setting . Any ideas how to fix it.
Setup ventri .211
Plug thunderbolt big bore
Fuel 10sny/10castor/10nitro
No muffler pressure
Cheers Steve
OY! A.211 venturi with no spigot? That's rather large, FAR bigger then I ever ran. I used a #5 drill (.205) with a 1/16x.030 spigot. With a spigot, maybe a .211/#3, but without, wow. You didn't mention a diffuser (air filter or pantyhose), that might give you a chance with that large a venturi and a flush outlet. I think it comes with a #12 venturi (.189) which normally gives very good power and will provide much better fuel suction.
However, an excessively-large venturi normally causes it to go lean, not rich. This is not the effect of a uniflow leak, which usually turns it into a suction tank, nor the venting (which is moot aside from windy weather). It *does* sound like the fuel getting heated but you did most of what it needed to combat that. Anything wrong will be greatly exaggerated with the large venturi, but going to a normal venturi will only reduce the issue, not solve it.
Other than restoring a normal setup with a normal-sized venturi (like a #7 or 8 with no spigot, and maybe #5 with a spigot and diffuser), the only thing I can think is that there is a heat-variable leak in the intake area somewhere. For example, the spraybar is leaking air around the screws and nuts holding it in that clears itself up as the engine temperature changes. This is pretty common if you do not seal your spraybar to the case with sealant, although usually that causes instability on the ground more than in the air. At least make sure to verify that the nut holding the spraybar and the collet are *tight*.
Maybe also an air leak around the venturi itself. The PA series is somewhat more prone to that than others because the compression of the o-ring around the venturi and holding it on the back makes it want to tilt in the bore. We usually selected for venturis with the smallest clearance fore/aft and it's important to use the stock *black* o-rings, or the venturis with the undercut if you use the red silicone types. I have seen a lot of people try a "pretty close" o-ring from the hardware store that was too thick to seal properly, or tended to squirt out from under the flange under pressure. The same things happened with the red/orange o-ring, until the bore was undercut to permit proper clearance. Too thick also exacerbates the tendency of the venturi to tilt in the bore.
Any of those could be it, or something else. This is a strange case, any time I saw this it was the fuel heating up.
Brett